In truth I'm not 100% sure what caused me to pick an F avatar over a M avatar. As it turns out, as others have mentioned, there are some benefits such as lower markup on some armours and a more pleasant third person viewpoint. But I didn't know about the former when I created the account and I don't really think I thought much latter either.
I think ultimately I just didn't really see it as all that big a deal. I was signing up to a sci-fi role-playing game and thought it'd be interesting to have a female avatar. I didn't see Oni as a reflection of me, I saw her as a character I could develop much like an author develops characters in a book (I have two other Onis now in other MMOs hehe).
There are a couple of downsides though. Firstly, you get to experience first-hand what dickheads some people of my gender can be. Only
some, but in my five years in EU I've really had to cringe at some of us guys. Secondly, although I never saw Oni as being 'me', other people do. At first that didn't bother me, I saw Oni as a character and other avatars as characters. But after you start to get to know people in game, on the forum and in RL, it's quite clear that a few people (some M but also some F) are made to feel awkward by the fact my avatar is a She but they know I am a He. So eventually I largely dropped the whole role-playing thing; MA destroyed the believability of EU from that point of view a long time ago anyway. I get called 'Oni' and 'Jimmy' from people I know in-game now, either is fine with me.
For that reason if I was starting over I might choose an M avatar instead. Although I'm still quite attached to Oni even if I don't view EU as role-playing so much these days.
It's a pretty safe bet to say gay men aren't interested in the female form.
I think it'd be a losing bet. My second girlfriend at uni was quite, um, busty and also had quite a lot of gay friends. And the male ones were
obsessed with her boobs lol, seriously.